1. Field of the Invention
This invention relates generally to methods for producing polyester containers and particularly to methods for determining the temperature distribution throughout the thickness of a polyester preform used in a reheat stretch blow molding process for making polyester containers.
2. Description of the Prior Art
The containers used to package many consumer products are made from polyester, particularly poly(ethyleneterephthalate) (“PET”). The process for producing such containers involves making a PET preform and reheat stretch blow molding the preform into a container, typically a bottle. These containers must have properties that permit them to function in the product manufacturing or packaging process and be capable of maintaining the integrity of the product in the container for prolonged periods. The container properties of most interest are typically the strength of the container, the permeability of the container to liquids and gases, and the haze (optical properties) of the final bottle. All of these parameters are a strong function of the preform temperature distribution at the time of blow molding. Using carbonated soft drinks as an example, the high pressure of CO2 in the container tends to cause the bottle to undesirably expand or “creep” over time. Similarly, the CO2 tends to permeate through the bottle sidewall until a point is reached where the soda goes “flat.” Both the creep resistance and the gas barrier can be improved by blowing the bottle at a colder temperature which improves the molecular orientation and strain-induced crystallization of the polyester chains. However, too cold of a stretch temperature and localized microtearing or “pearlescense” will occur.
The final properties of a stretch blow molded container are a strong function of the blow molding temperature. Because the inside and outside surfaces of a preform undergo differing degrees of stretch as the container is produced, it is important to optimize the blow temperature at both surfaces as well as through the center. For example, the inside surface of a typical preform might undergo a planar stretch ratio (“PSR”) of 11 whereas the outside surface PSR is only 9. Since the inside surface is stretched more than the outside surface, the inside surface should preferably be a few degrees hotter at inflation to prevent pearlescence (microtearing due to cold stretching) and to optimize the overall molecular or chain orientation.
Unfortunately, the ability to optimize the blow molding temperature has been hindered by the inability to determine the through-the-thickness temperature profile of the preform, i.e., the temperature at the inside and outside surfaces as well as the temperature throughout the preform body. Most reheat stretch blow molding machines (“RHB”) are equipped with, at most, a standard infrared (“IR”) pyrometer that measures only the outside surface temperature of the preform. Unfortunately, the inside surface temperature of the preform is actually more important for determining bottle properties. If too low, the inside surface will micro-tear during stretching resulting in a hazy bottle. If too hot, the inside surface will not strain-harden properly resulting in poor mechanical properties, e.g., reduced oxygen barrier.
With some effort, a preform can be removed from the blow molding process and a probe inserted inside the preform to obtain a rough estimate of the inside surface temperature. Unfortunately, this method is not very accurate. Also, this approach does not give the full temperature profile over the whole preform thickness and height. Furthermore, this approach is not conducive to real-time measurement and control because the preform must be removed from the manufacturing process for testing. The best alternative is to run costly trial-and-error type blow molding trials where the heating lamps are systematically varied until the optimal bottle properties are obtained. Unfortunately, this procedure is time consuming and expensive. More often than not, the blow molding process will be conducted at whatever conditions meet the minimum fitness for use specifications for a particular container, whether or not these are truly optimal for producing the highest quality containers.
There is, therefore, a need for a method and device for determining the temperature distribution throughout the thickness of a preform, particularly in real-time, and using such temperature distribution data to improve the container manufacturing process and ensure that only the highest quality containers are produced.